Antiglomerular antibodies have recently been found in immunoglobulin eluted from kidneys of rats with Heymann's nephritis. To demonstrate a role for these antibodies in the pathogenesis of Heymann's nephritis, paired-label radioisotope studies were used in the current study to quantitate binding of the eluted antibody to glomeruli of isolated perfused rat kidneys. In this perfusion situation, which largely excludes the formation of circulating immune complexes, specific binding of 0.9 to 3.2 per cent of the total eluate protein infused was found with 9 to 33 per cent of the bound protein recovered in the glomerular fractions. Specific glomerular binding was also observed after administration of the paired-label mixture to intact rats. Glomerular immunoglobulin deposits and subepithelial electron-dense deposits similar to those found in rats with Heymann's nephritis were produced by intravenous administration of the eluted antibody. Direct bonding of antibody to glomerular capillary wall antigens, in the manner similar to that established for antiglomerular basement membrane antibody, must be considered in the immunopathogenesis of Heymann's nephritis and potentially in some forms of human glomerular injury as well.